<template>
	<ul class="list1">
		<li 
      class="item" 
      v-for="item in letters" 
      :key="item" 
      :ref="item"
      @click="handleLetterClick"
      @touchstart="handleTouchStart"    
      @touchmove="handleTouchMove" 
      @touchsend="handleTouchEnd" 
    >{{item}}</li>
	</ul>
</template>


<script>
	export default{  
		props:{
			cities:Object
		},
    data(){
      return{
        touchStatus:false,
        startY:0,
        timer:null      //数据节流
      }
    },
    updated(){
      this.startY=this.$refs['A'][0].offsetTop
    },
    computed:{
      letters(){
        const letters=[]
        for(let i in this.cities){
          letters.push(i)
        }
        return letters      //得到一个数组 ["A","B","C"...]
      }
    },
    methods:{
      handleLetterClick(e){
        this.$emit('change',e.target.innerText)
      },
      handleTouchStart(){         //手指按下
        this.touchStatus=true
      },
      handleTouchMove(e){          //滑动中
        if(this.touchStatus){        //A 距顶距离
          if(this.timer){
            clearTimeout(this.timer)
          }
          this.timer=setTimeout(()=>{
            const touchY=e.touches[0].clientY-79                 //手指按下时，距顶的距离
            const index=Math.floor((touchY-this.startY)/20)       // 20 为每个字母高度 得到当前滑动位置对应的下标
            if(index>=0&&index<this.letters.length){
              this.$emit('change',this.letters[index])
            }
          },16)
        }
      },
      handleTouchEnd(){           //手指抬起

      }
    }
	}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list1
    display: flex
    flex-direction: column
    justify-content: center
    position:absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
</style>